<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    console.log(!true);//false
    console.log(true && false);//false
    console.log(true || false);//true

    var a="",
        b="B有值";
    var result=a||"A的值是假的",
        result2=b||"B的值是假的";
    console.log(result);
    console.log(result2);
</script>
</body>
</html>